Determining fatigue response of an electric motor component

Abstract

A method of determining fatigue response of an electric motor component in an electric motor, comprising: obtaining S-402 a first finite element model of the electric motor; obtaining S-404 a data dis

Description

TECHNICAL FIELD The present disclosure relates to determining a fatigue response of an electric motor component. Aspects of the invention relate to a computer implemented method, computer-readable instructions, a system and a vehicle including an electric motor. BACKGROUND It is known to provide vehicles having an electric powertrain, for example hybrid vehicles, Plug-in Hybrid Electric Vehicles (PHEV), and Battery Electric Vehicles (BEV). Such vehicles utilise an electric motorto provide traction power. In order to assess the condition of components within the electric motor, the motor is typically at least partially disassembled such that the components can be inspected. This may be done at regular intervals so that the evolving condition of the electric components can be monitored. This process is labour-and time-intensive, and the vehicle cannot be driven while the inspection is in progress. In different contexts, it is known to provide a “digital twin” of a mechanical component for the purpose of modelling and monitoring the condition of the mechanical component. Such digital twins typically use a Stress-Life (or “S-N”) analysis to predict a condition of the mechanical component. However, such methods may provide only limited accuracy in predicting the condition of the mechanical component, and are not necessarily optimal for monitoring the condition of components in a vehicle electric traction motor. It is an aim of the present invention to address one or more of the disadvantages associated with the prior art. SUMMARY OF THE INVENTION Aspects and embodiments of the invention provide a computer implemented method, computer-readable instructions, a system and a vehicle as claimed in the appended claims. According to an aspect of the present invention there is provided a computer implemented method of determining fatigue response of an electric motor component (for example a stator lamination or a rotor lamination) in an electric motor, the method comprising: obtaining a Strain-Life function configured to calculate a value indicative of a fatigue response of the electric motor component. The method further comprises, fora first time period: receiving data indicative of one or more conditions (for example temperature, output torque or rotational speed) of the electric motor during the first time period; determining strain values for at least a part of the electric motor component; and using the Strain-Life function to determine a first fatigue response value for the at least a part of the electric motor component based on the determined strain values. Optionally the fatigue response value is, or is used to determine, a fatigue life of the electric motor component. According to an aspect of the present invention there is provided a computer implemented method of determining fatigue response of an electric motor component (for example a stator lamination or a rotor lamination) in an electric motor, the method comprising: obtaining a first finite element model of the electric motor (for example of the whole motor or a part or section thereof); obtaining a data discretisation function configured to discretise data relating to operating conditions (for example temperature, output torque or rotational speed) of the electric motor; and obtaining a Strain-Life function (optionally comprising a range-mean Rain flow matrix, for example in combination with counting algorithm for populating the matrix) configured to calculate a value indicative of a fatigue response of the electric motor component. The method further comprises, for a first time period: receiving data indicative of one or more conditions (for example temperature, output torque or rotational speed) of the electric motor during the first time period; discretising the received data during the first time period using the data discretization function; using finite element analysis to calculate strain for a plurality of elements (optionally all elements) of the first finite element model based on the discretized data; determining strain values for at least one element of the plurality of elements of the first finite element model, the at least one element of the first finite element model corresponding to a part of the electric motor component; and using the Strain-Life function to determine a first fatigue response value for the at least one element of the first finite element model based on the determined strain values. Optionally the fatigue response value is, or is used to determine, a fatigue life of the electric motor component. Advantageously, the present invention provides an electric motor-specific “digital twin” that provides an indication of the fatigue condition of an electric motor component (and optionally more than one electric motor component), without the need to disassemble and inspect the electric motor and its components. Furthermore, the combination of discretising the data and using finite element analysis to extract strain, and then using a Strain-Life (or“E-N”) function provides improved accuracy in fatigue response determination. Thus, the present invention provides an accurate and effective means for monitoring the fatigue response / condition of electric motor components. Optionally, the first finite element model is based on measured or otherwise determined physical properties of the electric motor. This advantageously produces a “digital twin” that is bespoke to an individual electric motor, and thereby provides further enhanced accuracy. Optionally, the method comprises, for each of one or more further time periods: receiving further data indicative of the one or more conditions of the electric motorduring each of the respective furthertime period; discretising the further data using the data discretization function; using finite element analysis, calculating strain for the plurality of elements of the first finite element model based on the discretized further data; determining further strain values for the at least one element of the first finite element model; and using the Strain-Life function, determining an updated (or cumulative) fatigue response value for the at least one element of the first finite element model based on the determined further strain values and the first fatigue response value. Beneficially this provides an iterative process allowing for the evolving fatigue response of the electric motor component to be accurately monitored overtime as the electric motor is operated. Optionally the method comprises determining if the first (or updated / cumulative) fatigue response value has reached a threshold fatigue response value; and in dependence on determining that the first fatigue response value has reached the threshold fatigue response value, generating a first signal indicative that the first fatigue response value has reached the threshold fatigue response value. Advantageously, the first signal provides an indication that the electric motor component needs, or will soon need maintenance, repair or replacement. In an embodiment, in dependence on receiving the first signal a vehicle display is configured to display content indicative that the electric motor component requires maintenance or replacement. In a further embodiment, in dependence on receiving the first signal, a vehicle powertrain controller is configured to modify operation of the electric motor. Beneficially, the invention allows for remedial action to be taken to operate the electric motor to reduce further fatigue in the electric component prior to its repair or replacement, either by means of prompting the driver, or dynamically and automatically through interaction with the powertrain controller. Optionally the method comprises determining, based on the first fatigue response value and the updated / cumulative fatigue response value, a rate of change of fatigue response value; determining if the rate of change of fatigue response value exceeds a threshold rate; and in dependence on determining that the rate of change of fatigue response value exceeds the threshold rate, generating a second signal indicative that a current operation of the motor is causing accelerated fatigue. In an embodiment, in dependence on receiving the second signal a vehicle display is configured to display content providing a user driving suggestion. In a further embodiment, in dependence on receiving the second signal, the vehicle powertrain controller is configured to modify operation of the electric motor. Beneficially, the invention allows for remedial action to be taken to operate the electric motor to alert the driver to operation of the electric motor that may result in premature fatigue in electric motor components and suggest a way of operating the vehicle / electric motor to reduce premature fatigue, and / or dynamically and automatically interacting with the powertrain controller to modify operation of the electric motor to avoid or reduce premature fatigue. Optionally the method comprises diagnosing a maintenance condition of the electric motor based on the first fatigue response value. Advantageously, the present invention allows maintenance personnel / equipment to identify potential issues based on the predicted fatigue response without having to disassemble the electric motor and examine every component. In an embodiment, the method comprises, priortothe first time period: obtaining a second model of the electric motor; obtaining predetermined simulation data indicative of one or more simulated operating conditions of the electric motor; and using the second model and the predetermined simulation data to identify at least one location / part on the electric motor component corresponding to elements of the plurality of elements in the second finite element model having a simulated fatigue response value above a threshold simulated fatigue response value; wherein the at least one element of the first finite element model corresponds to the identified at least one location. Beneficially, only the elements of the first finite element model corresponding to locations / parts of the electric motor component most likely to experience fatigue are analysed / monitored - this reduces the computation resource required to perform the method. According to another aspect of the present invention there is provided computer readable instructions which, when executed by a computer, are arranged to perform the method above. According to yet another aspect of the present invention there is provided a system comprising one or more processors collectively configured to perform the method above. In an embodiment, the system comprises at least one electronic processor having an electrical input for receiving an input signal; and at least one memory device electrically coupled to the at least one electronic processor and having instructions stored therein; and wherein the at least one electronic processor is configured to access the at least one memory device and execute the instructions thereon so as to perform the method above. According to a further aspect of the present invention there is provided a vehicle comprising: the system above and the electric motor, wherein the electric motor comprises the electric motor component. In an embodiment, the vehicle (for example via a user interface of the vehicle) is configured to, in dependence on receiving the first signal the display, display content indicative that the electric motor component requires maintenance or replacement; and in dependence on receiving the second signal, the display is configured to display content providing a user driving suggestion, as discussed above. In an embodiment, the vehicle comprises a powertrain controller, wherein: in dependence on receiving the first signal or the second signal, the powertrain controller is configured to modify operation of the electric motor, as discussed above. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in anyway and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. BRIEF DESCRIPTION OF THE DRAWINGS One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 shows a representation of a vehicle in accordance with embodiments of the present invention; Figure 2 shows a schematic representation of components of the vehicle of figure 1; Figure 3 shows a schematic representation of a rotor lamination; and Figure 4 shows a flow chart representing a method for monitoring electric motor component condition in accordance with an embodiment of the present invention. DETAILED DESCRIPTION A vehicle 10 in accordance with an embodiment of the present invention is described herein with reference to the accompanying Figures 1 and 2. With reference to Figure 2, the vehicle 10 of Figure 1 is illustrated schematically. The vehicle 10 includes a monitoring system 100 and an electric powertrain 120. The electric powertrain 120 comprises a least one electric motor 122. The electric motor 122 is an electric motor arranged to convert electrical energy into kinetic energy in the form of mechanical torque and is also arranged to convert kinetic energy in the form of mechanical torque into electrical energy (for example during regenerative braking). The electric motor 122 may be an alternating current induction motor or a permanent magnet motor, or another type of suitable electric machine. The electric motor 122 is a traction motor to drive the vehicle 10. Another term for the electric motor 122 is an electric drive unit (EDU). The electric motor 122 is configured to drive at least one vehicle wheel 20 (typically at least two wheels 20, and optionally four wheels 20 as illustrated), for example via a transmission output shaft 124 and one or more sets of gears 126 (for example one or more differentials or transaxles) as shown in figure 2. In alternative embodiments, a plurality of electric machines 122 may be provided, with each electric machine driving a respective wheel 20 or axle. The electric powertrain 120 optionally further comprises a powertrain controller (also referred to as a motor controller) 128 configured to control operation of the electric motor 122. The vehicle 10 also comprises an electrical energy storage 130, for example a traction battery. The electrical energy storage 130 is configured to deliver electrical energy to the electric motor 122 when the electric machine is driving one or more wheels 20, and to receive and store electrical energy generated by the electric machine during regenerative braking. The electric motor 122 is electrically connected to the electrical energy storage 130, optionally by an inverter (not shown). In some embodiments, the electrical energy storage 130 is communicatively coupled to the powertrain controller 128. The electrical energy storage 130 is optionally a high voltage battery. The traction battery 130 may have a voltage and capacity to support electric only driving for sustained distances. The traction battery 130 may have a capacity of several kilowatt-hours, to maximise range. The capacity may be in the tens of kilowatt-hours, or even over a hundred kilowatt-hours. The electric machine comprises a rotor assembly 140 and a stator assembly 142. As illustrated in figure 2, the electric motor 122 comprises one or more sensors 144, 146,148. Alternatively, the one or more sensors 144, 146, 148 may be provided separately to the electric motor 122. The one or more sensors 144, 146, 148 are configured to measure one or more operating conditions of the electric motor 122. In an embodiment: a first sensor 144 is configured to measure a temperature of the electric motor 122, the stator assembly 142, the rotor assembly 140, and / or a component thereof as a function of time; a second sensor 146 is configured to measure an output torque of the electric motor 122 as a function of time; and a third sensor 148 is configured to measure a rotational speed of the rotor assembly 140 or other rotating part of the motor 122, for example relative to the stator assembly 142, as a function of time. Alternatively, a subset of the first sensor 144, second sensor 146 and third sensor 148 may be provided. As known in the art, stators and rotors of electric machines commonly comprise a number of components. The type, number and configuration of the rotor and stator components vary, and depend, for example on the end application and performance requirements of the electric machine, whether the electric machine operates on a DC or AC input, whether the electric machine employs permanent magnets or inductive means, etc. The present electric motor 122 comprises one or more electric motor components. The one or more electric motor components may include a rotor lamination, a rotor permanent magnet pole piece, a rotor endcap, a rotor pushnut, a stator lamination, a rotor shaft, and / or a rotor shaft bearing. It will be appreciated that the one or more electric motor components may include additional or different elements. As an example of an electric motor component, figure 3 shows a schematic top view of a rotor lamination 160 comprising a plurality of apertures 162. In one example, rotor lamination 160 is one of a stack of similar laminations positioned in between permanent magnet pole pieces and coupled to a rotor output shaft. Returning to Figure 2, the monitoring system 100 comprises processing means 102 and memory means 103. The processing means 102 may be one or more electronic processing device 102 which operably executes computer-readable instructions. The memory means 103 may be one or more memory device 103. The memory means 103 is electrically coupled to the processing means 102. The memory means 103 is configured to store instructions, and the processing means 102 is configured to access the memory means 103 and execute the instructions stored thereon. Optionally the monitoring system 100 is, or is part of, the powertrain controller 128. In the illustrated embodiment, the monitoring system 100 comprises an input means 104 and an output means 105. The input means 104 may comprise an electrical input of the monitoring system 100. The output means 105 may comprise an electrical output of the monitoring system 100. The input means 104 is arranged to receive signals from other components in the vehicle 10, for example the one or more sensors 144, 146, 148. The input means 104 may also be arranged to receive signals from outside the vehicle 10, for example wirelessly. In some embodiments, the received signals are electrical signals indicative of, for example, temperature, output torque and rotational speed for the electric motor 122, as described in more detail below. The output 105 is arranged to output signals (for example control signals), for example electrical signals, as described in more detail below. In some embodiments, vehicle 10 includes a user interface 150 configured to display content to one or more drivers / occupants or users of the vehicle 10 via a display 152. The user interface 150 may form part of an instrument panel ordashboard display, or part of an infotainment system. Figure 4 illustrates a method 400 according to an embodiment of the invention. The method 400 is a method of determining fatigue response of an electric motor component in an electric motor. The method 400 may be performed at least in part by the monitoring system 100 illustrated in Figure 2. In particular, the memory 103 may comprise computer-readable instructions which, when executed by the processor 102, perform the method 400 according to an embodiment of the invention. The method 400 starts at step S-402, in which a first finite element model (FEM) of the electric motor 122 is obtained. In some embodiments the first finite element model describes the entirety of the electric motor 122. Alternatively, the first finite element model describes a portion of the electric motor 122, such as the rotor assembly 140, orthe stator assembly 142. As will be appreciated to one skilled in the art, each element of the first FEM corresponds to a part (more specifically a location on a part) of the electric motor 122. For example, one or more elements correspond to the electric motor component being monitored for fatigue response, such as rotor lamination 160. The first finite element model is generated based on the properties of the specific electric motor 122 (or rotor assembly 140, or stator assembly 142) being monitored. In one example, the finite element model is based on one or more of: the measured component geometry, dimensions and clearances, for example the tolerance in the dimensions of a magnet slot or in the dimensions of a rotor shaft and rotor lamination interference fit feature; the measured / determined surface roughness; the measured / recorded number of stator winding turns; the measured magnetic field strength of permanent magnet pole pieces; the material properties of at least one (for example each) electric motor component, and measured or otherwise determined non-linearities in the material properties of the at least one electric motor component. Advantageously, by using a first FEM that is specific to the properties of the specific electric motor 122 being monitored, a more accurate determination of fatigue response of the electric motor component is made. Once the properties of the specific electric motor 122 have been measured, identified or otherwise determined, the FEM can be generated using principles for FEM generation known in the art. In one embodiment, the first FEM is generated during / after assembly of the electric motor 122, and optionally before the vehicle 10 has been driven for the first time and / or been delivered to an end user. The FEM is then stored in memory means 103 and / or in other computer readable storage accessible to the monitoring system 100, for example cloudbased storage (not shown). In some embodiments, obtaining the first FEM at step S-402 involves retrieving or accessing, by the processing means 102, the first FEM from the memory means 103 or cloud-based storage. At step S-404, a data discretization function is obtained. As described in more detail below, data describing electric motor 122 operating conditions (e.g., temperature, output torque and rotational speed) are received from the one or more sensors 144,146,148 by the monitoring system 100. To use the data in the subsequent fatigue response determination, it is first discretized using the discretization function, as described in more detail below. In some embodiments, the data discretisation function comprises a range-mean Rain flow matrix and a counting algorithm. The range-mean Rain flow matrix is a square matrix of dimension n. The counting algorithm is, for example, the Rain flow cycle counting algorithm as defined in standard ASTM E1049-85, which is configured to populate the range-mean Rain flow matrix using input data, thereby resulting in a matrix of discretized data. A separate range-mean Rain flow matrix is provided for each data source. For example, a first range-mean Rain flow matrix is provided for temperature data received from the temperature sensors 144, a second range-mean Rain flow matrix is provided for output toque data received from the output torque sensors 146, and a third range-mean Rain flow matrix is provided for rotational speed data received from the rotational speed sensor 148. The dimension of the / each range-mean Rain flow matrix is advantageously determined so as to enable highly accurate fatigue response determinations while reducing the computational resource demand (e.g., processing and memory resource) at the vehicle 10 / monitoring system 100. Each range-mean Rain flow matrix corresponds to the full range of expected data, for example all values between an expected minimum temperature / output torque / rotational speed and a respective expected maximum temperature / output torque / rotational speed. For centrifugal speed loading, each range-mean Rain flow matrix spans an entire motor speed range from zero to the maximum speed of the electric motor 122. Each full range is divided into bins each corresponding to a sub-range of the full range. The number of bins fora particular value corresponds to the dimension n of the respective range-mean Rain flow matrix. The inventors have found that a bin size of 10 degrees C for temperature, 20 Nm for output torque and 500 RPM or 100 RPM for rotational speed provide accurate fatigue response determinations, without placing undue computational burden on the monitoring system 100 / vehicle 10. In some embodiments, the data discretization function is stored in memory means 103 before the vehicle has been driven forthe first time and / or been delivered to an end user. In this case, obtaining the data discretisation function at step S-404 involves retrieving, by the processing means 102, the data discretisation function from the memory means 103. At step S-406, a Strain-Life function (also referred to as an “E-N function” or “s-N function”) is obtained. In one embodiment, the Strain-Life function comprises a Coffin-Manson-Basquin Strain-Life model, optionally further including a Miner’s linear damage accumulation rule (also known in the art). Advantageously, this technique can account for any surface treatment performed on the electric motor component prior to assembly of the electric motor 122. In some embodiments, the Strain-Life function is stored in memory means 103 before the vehicle 10 has been driven forthe first time and / or been delivered to an end user. In this case, obtaining the Strain-Life function at step S-404 involves retrieving or accessing, by the processing means 102, the Strain-Life function from the memory means 103. The Strain-Life function is configured to output a fatigue response value. The fatigue response value is indicative of the condition of the electric motor component or a part thereof. The fatigue response value may be, or may be used to determine a remaining fatigue life, typically a dimensionless value between 0 and 1 which is a measure of how close the electric motor component is to developing a crack. A value of 1 indicates a crack has appeared. It will be appreciated that steps S-402, S-404 and S-406 above may be performed in any order. Using the obtained first FEM, the data discretization function and the Strain-Life function, the monitoring system 100 is configured to determine the fatigue response of at least one electric motor component (such as rotor lamination 160). In one embodiment, a cumulative fatigue response is determined over one or more time periods while the vehicle 10 is being driven. Each time period may a fixed predetermined length of time, for example 15 minutes, 1 hour, 5 hours, etc. Alternatively, each time period may correspond to a time taken for the vehicle 10 to travel a predetermined distance, for example 20 km, each time period thus being dependent on the speed of travel of the vehicle 10 during driving. Other methods for determining the time period may also be used. For a first time period (for example during the first time period or subsequent to the first time period), the method performs steps S-408, S-410, S-412, S-414 and S-416 as described below. At step S-408, the monitoring system 100 receives (for example from the one or more sensors 144, 146, 148) data indicative of one or more conditions of the electric motor 122 (for example temperature, output torque and / or rotational speed) experienced during the first time period. The one or more sensors 144, 146, 148 are configured to measure respective conditions of the electric motor 122 multiple times throughout the first time period (for example once every 10 seconds, 1 second, or multiple times per second). Thus, the monitoring system receives, at step S-408, a plurality of data values for each respective condition of the electric motor. At step S-410, the monitoring system 100 (for example using processing means 103) uses the data discretization function to discretize the data received in step S-408. For example, a Rain flow cycle counting algorithm may be used to discretize temperature data from temperature sensor 144, into a corresponding range-mean Rain flow matrix for temperature having dimensions corresponding to a bin size of 10 degrees C. Range-mean Rain flow matrices for other conditions such as output torque and rotational speed may be similarly populated. At step S-412, the monitoring system 100 is configured to perform finite element analysis (FEA) using the first FEM and the discretized data obtained in step S-410 (for example using the data from one or more of a temperature range-mean Rain flow matrix, an output torque range-mean Rain flow matrix and a rotational speed range-mean Rain flow matrix), to calculate a strain corresponding to a plurality of elements of the first FEM. The FEA is performed using known techniques. Thus, the strain experienced by the electric motor component is modelled. In some embodiments, the stress experienced by the electric motor component is also modelled, and at step S-412 stress is also calculated for the plurality of elements of the first FEM using FEA and the discretized data. At step S-414, the monitoring system 100 determines strain values (for example Logarithmic Strain, True Strain, and / or Plastic Strain) for at least one element of the first FEM. As noted above, each element in the first FEM corresponds to a part / location in the electric motor 122. In one embodiment, strain is calculated for only a subset (at least one) of the total number of elements present in the first FEM at step S-412. Advantageously, this reduces the computational demand required when performing fatigue response monitoring. The at least one element of the FEM is chosen based on a corresponding part of the electric motor component (e.g., rotor lamination 160) expected to experience higher / the highest levels of strain and / or higher / the highest levels of fatigue response relative to other parts of the electric motor component. Optionally, such elements are identified before the vehicle 10 is first driven (for example during manufacture) using either the first FEM, or a second FEM also corresponding to the electric motor 122 but created using estimated properties of the electric motor rather than specifically determined / measured properties. In this case, predetermined simulation data is discretized according to the discretization function, the strain in every element of the first (orsecond) FEM is calculated using FEA, and the Strain-Life function is used to calculate a simulated fatigue response of each element in the first (or second) FEM. The predetermined discretization data optionally corresponds to a continuous maximum expected / possible temperature, output torque, rotational speed, or other electric motor condition applied over a predetermined period of time. The predetermined simulation data therefore thus does not represent a real world situation - instead it represents a simulated “stress test” of the electric motor 122. The determined simulated fatigue responses are then compared to a predetermined threshold. All elements that have a simulated fatigue response above the predetermined threshold are selected as the one or more elements for which the strain is calculated during step S-412 during real-world monitoring of the electric motor component. Alternatively, or in addition, the one or more elements can be limited to elements corresponding to a surface of the electric motor component - in general, cracks in objects caused by fatigue typically initiate at, and propagate from, the surface of the object. The identification, and determination of the strain, ofthe subset of elements therefore allows forthe monitoring system 100 to determine the strain forthe elements which are most likely to suffer from fatigue, and therefore require monitoring. In this manner, a determination of fatigue for the electric motor and the electric motor components can be made whilst minimising the number of calculations, and thus the computational requirement, to determine fatigue. Returning to the example of figure 3, two parts / locations / regions 164a, 164b of rotor lamination 160 are illustrated, the parts 164a, 164b corresponding to first (or second) FEM elements having a simulated fatigue response above the predetermined threshold. In this example, elements in the First FEM corresponding to these parts 164a, 164b are actively monitored by the monitoring system 100 during driving ofthe vehicle 10, i.e., the strain is calculated forthe elements corresponding to these parts 164a, 164b at step S-412. In other embodiments, a different model may be used for the purpose of determining which one or more elements are chosen to calculate strain values for. In embodiments where stress has been calculated at step S-412, step S-414 also includes determining stress values forthe only or more elements ofthe first FEM. Returning to figure 4, once the strain values have been calculated for the one or more elements of the first FEM, the Strain-Life function is used to determine a first fatigue response value forthe at least one element using the strain values at step S-416. Optionally, the average surface roughness is taken as an additional process parameter to account for the roughness effect on the fatigue response. The Strain-Life function is optionally solved analytically to obtain the fatigue response value, for example using the Newton-Raphson method known in the art. Optionally, the fatigue response value is, or is used to calculate, a remaining fatigue life ofthe electric motor component. In some embodiments, the determined fatigue response value is then stored in memory means 103 or in alternative storage. In some examples, prior to calculating the fatigue response value, the strain values (and, if present, stress values) are further processed using the signed von Mises combination method known in the art. Advantageously, this combination method allows forthe stress state ofthe element i.e., whetherthe von Mises stress is compressive or tensile to be accounted for, further enhancing the accuracy of the fatigue response determination. Optionally, a mean stress correction (for example using the Smith-Watson Topper Mean Stress Correction, or alternatively the Morrow Mean Stress Correction) is also applied prior to calculating the fatigue response value. Though described in relation to a particular electric motor component (e.g., rotor lamination 160), it will be appreciated that the condition of several electric motor components can monitored in this way. For example, where the first FEM describes the whole electric motor 122, first fatigue response values can be calculated for each of a plurality of components within the electric motor 122 at step S-416. Advantageously, the present invention thus provides a means for monitoring the condition of an electric motor component without the need to physically inspect the electric motor component. The invention provides an accurate determination of any damage that may be experienced by the electric motor component during normal use of the electric motor 122 while driving the vehicle 10, without the need to disassemble the electric motor 122. This in turn may be used to diagnose faults, identify when pre-emptive maintenance is required, and equally avoid unnecessary inspection of electric motor components. Furthermore, by analysing strain to determine a fatigue response (in particular via FEA applied to discretized data), the invention provides a more accurate determination of the condition of the electric motor component, for example more accurately accounting for factors such as surface roughness. This is in contrast to known techniques for modelling the condition of mechanical items in other contexts, which are based only on stress values estimated by scaling input loads (also called the Stress-Life or “S-N” model). In some embodiments, steps S-408, S-410, S-412, S-414 and S-416 are repeated for subsequent time periods. As noted above, the subsequent time periods may be the same length as the first time period, or may be different lengths of time (e.g., where all time periods correspond to a predetermined distance travelled since the start of the time period). In this case, for each consecutive time period at step S-416 the Strain-Life function is used to determine an updated or cumulative fatigue response, indicative of the cumulative effects of the strain experienced by the electric motor component. Thus, the present invention may provide an iterative process, where the evolving condition of the electric motor component is tracked overtime. Optionally, after step S-416 has been performed one or more times, it is determined whether the fatigue response value (or updated / cumulative fatigue response value) has reached a threshold fatigue response value in step S-418. If the threshold fatigue response value has been reached, a first signal is generated at Step S-420, the first signal indicative that the threshold fatigue response value has been reached. The threshold fatigue response value may be chosen to correspond to a condition of the electric motor component wherein damage (e.g., a crack) has occurred, and / or damage (e.g., a crack) has not yet occurred, but is predicted to occur (for example within a certain time frame or distance travelled by the vehicle 10). In some examples, the first signal is generated by the monitoring system 100 and transmitted to user interface 150 at step S-422. In response, the user interface 150 displays a warning to the driver of the vehicle 10, indicating that maintenance of the electric motor 122 (for example maintenance or replacement of the electric motor component) is required. Alternatively or in addition, the first signal is sent to the powertrain controller 128, and at step S-424, the powertrain controller 128 is configured to modify the operation of the electric motor 122 so as to reduce a rate of change of fatigue response of the electric motor component. The modification may, for example involve reducing an output torque or output speed delivered by the electric motor 122 as compared to a demanded output torque or output speed corresponding to a driver input. The modification may be kept in place until maintenance on the electric motor122 / electric motor component has been completed. Optionally the first signal may be transmitted to a maintenance location to provide advance notice of maintenance that may be required on the electric motor 122 / electric motor component. Advantageously, the present invention thus allows for real-time feedback to be provided to the driver during operation of the vehicle regarding the condition of electric motor components, whose condition would otherwise need to be determined by disassembly of the electric motor 122 and inspection of the motor components. Furthermore, remedial action can be taken dynamically and automatically by the vehicle by modifying the response of the motor to driver instructions to preserve the electric motor components until maintenance can be performed. Optionally, following at least two iterations (i.e., at least one repetition) of steps S-408, S-410, S-412, S-414 and S-416, step S-426 is performed, in which a rate of change of the fatigue response value over time is calculated based on fatigue response values calculated at respective steps S-416. At step S-428, it is then determined if the calculated rate of change of fatigue response value exceeds a threshold rate. If the calculated rate of change of fatigue response value does exceed the threshold rate, a second signal is generated at step S-430, the second signal indicative that the electric motor 122 is being operated in a manner causing accelerated fatigue response (for example accelerated damage / deterioration in condition) in at least one electric motor component. In some examples, the second signal is generated by the monitoring system 100 and transmitted to user interface 150 at step S-432. In response, the user interface 150 displays a warning and / or a user driving suggestion to the driver of the vehicle 10. The user driver suggestion provides details of an alternative way of operating the vehicle 10 so as to reduce the rate of change of fatigue response value of the electric motor component (for example, recommending keeping the vehicle speed below a certain level, reducing acceleration where possible, etc.). Alternatively or in addition, the second signal is sent to the powertrain controller 128, and at step S-434, the powertrain controller 128 is configured to modify the operation of the electric motor 122 so as to reduce the rate of change of fatigue response of the electric motor component. The modification may, for example involve reducing an output torque or output speed delivered by the electric motor 122 as compared to a demanded output torque or output speed corresponding to a driver input. Such modification may be temporary, for example in place only until a temperature of the motor 122 has decreased, and / or the rate of change of fatigue response decreases to below the threshold rate. Optionally the second signal may be transmitted to a maintenance location to provide advance notice of maintenance that may be required on the electric motor 122 / electric motor component. Advantageously, the present invention thus allows for real-time feedback to be provided to the driver during operation of the vehicle regarding the condition of electric motor components, and allows the driver to take remedial action to increase the lifetime of electric motor components. Additionally, dynamic and automatic remedial action to increase the lifetime of electric motor components can be taken by the vehicle itself. In some examples, both the first and second signals can be generated as described above. Optionally, the fatigue response value (or the updated / cumulative fatigue response value) calculated at step S-416 can be used fordiagnosing maintenance conditions of the electric motor 122. For example, if a problem is reported with an electric motor 122, a maintenance engineer may interrogate the monitoring system 100 or otherwise retrieve the fatigue response values, and use the fatigue response values to identify any electric motor components that may have experienced damage for further investigation / repair / replacement. Advantageously, the present invention thus allows for efficient diagnosis of maintenance issues / conditions in electric motors, reducing or avoiding the need for a full inspection of every electric motor component, and / or alerting maintenance engineers to issues that may be difficult to detect using traditional measurement techniques. It will be appreciated that various changes and modifications can be made to the present invention without departing from the scope of the present application.

Claims

1. A computer implemented method of determining fatigue response of an electric motor component in an electric motor, the method comprising:obtaining a first finite element model of the electric motor;obtaining a data discretisation function configured to discretise data relating to operating conditions of the electric motor;obtaining a Strain-Life function configured to calculate a value indicative of a fatigue response of the electric motor component;for a first time period:receiving data indicative of one or more conditions of the electric motorduring the first time period;discretising the received data during the first time period using the data discretization function;using finite element analysis to calculate strain for a plurality of elements of the first finite element model based on the discretized data;determining strain values for at least one element of the plurality of elements of the first finite element model, the at least one element of the first finite element model corresponding to a part of the electric motor component; andusing the Strain-Life function to determine a first fatigue response value for the at least one element of the first finite element model based on the determined strain values.

2. The method of claim 1, further comprising determining a fatigue life of the electric motor component based on the determined fatigue response value.

3. The method of any preceding claim, wherein the data discretisation function comprises a rangemean Rain flow matrix.

4. The method of any preceding claim, wherein the first finite element model is based on measured properties of the electric motor.

5. The method of any preceding claim, wherein the one or more conditions of the electric motor comprises at least one of: a measured motor temperature, a measured rotational speed of a rotating part of the motor, and an output torque of the motor.

6. The method of any preceding claim, further comprising:for each of one or more further time periods:receiving further data indicative of the one or more conditions of the electric motorduring each of the respective further time period;discretising the further data using the data discretization function;using finite element analysis, calculating strain for the plurality of elements of the first finite element model based on the discretized further data;determining further strain values for the at least one element of the first finite element model; andusing the Strain-Life function, determining an updated fatigue response value for the at least one element of the first finite element model based on the determined further strain values and the first fatigue response value.

7. The method of any preceding claim, further comprising:determining if the first fatigue response value exceeds a threshold fatigue response value; andin dependence on determining that the first fatigue response value exceeds the threshold fatigue response value, generating a first signal indicative that the first fatigue response value exceeds the threshold fatigue response value.

8. The method of claim 6, further comprising:determining, based on the first fatigue response value and the updated fatigue response value, a rate of change of fatigue response value;determining if the rate of change of fatigue response value exceeds a threshold rate; andin dependence on determining that the rate of change of fatigue response value exceeds the threshold rate, generating a second signal indicative that a current operation of the motor is causing accelerated fatigue.

9. The method of any preceding claim, further comprising:diagnosing a maintenance condition of the electric motor based on the first fatigue response value.

10. The method of any preceding claim, further comprising, prior to the first time period:obtaining a second model of the electric motor;obtaining predetermined simulation data indicative of one or more simulated operating conditions of the electric motor; andusing the second model and the predetermined simulation data to identify at least one location on the electric motor component corresponding to elements of the plurality of elements in the second finite element model having a simulated fatigue response value above a threshold simulated fatigue response value;wherein the at least one element of the first finite element model corresponds to the identified at least one location.

11. The method of any preceding claim, wherein the electric motor component is a stator lamination or a rotor lamination.

12. Computer readable instructions which, when executed by a computer, are arranged to perform a method according to any of claims 1 to 11.

13. A system comprising one or more processors collectively configured to perform a method according to any of claims 1 to 11.

14. A vehicle comprising:the system of claim 13;the electric motor, the electric motor comprising the electric motor component.

15. The vehicle of claim 14 when dependent on claim 7 or claim 8 further comprising a display, wherein: in dependence on receiving the first signal the display is configured to display content indicative that the electric motor component requires maintenance or replacement; andin dependence on receiving the second signal, the display is configured to display content providing a user driving suggestion.

16. The vehicle of claim 14 when dependent on claim 7 or claim 8, further comprising a powertrain controller, wherein:in dependence on receiving the first signal or the second signal, the powertrain controller is configured to modify operation of the electric motor.17

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